It would be interesting to see what the permission slip said.
Maybe the school got around the Establishment Clause by saying this was a "cultural education". Shady. I could see it for Comparative Religion 101 at University of X, but for an elementary class? Huh?
There seems to be a blur between cultural and religious. That line should be made clear, so the parents can make an informed decision before signing the permission slip.
I'm not a parent yet. But if I was, and little silky was in this type of situation, I'd call the teacher and ask for the details and what the exact itinerary was going to be. Then I'd tell my kid to ask lots of questions. When the lil shit got back home, I'd ask how the day went and what s/he learned and liked and didn't like. It would be a good chance to see how they saw it, since that's the most important thing, and then straighten out any misunderstandings. I'm the parent.
And hopefully I'd be a good enough parent that I would be more influential to little silky than a few hours at a mosque or church or whatever.
I would not, repeat, would not spray the kid down with a pressure washer.